This invention relates to a reinforced aluminum matrix composite having improved toughness and ductility over known composites, without any significant sacrifice in strength or stiffness. In particular, the invention relates to a reinforced aluminum alloy consisting essentially of soluble amounts of copper and magnesium as the principal alloying elements. The alloy of the invention also may include other soluble alloying elements, alone or in combination, such as silicon, silver, or zinc, up to their solubility limits in the base alloy. Insoluble metallic elements, such as manganese, chromium, iron, and zirconium are eliminated or minimized.
Aluminum alloys are well-known and commonly used engineering materials. It is also well-known that incorporation of discontinuous silicon carbide reinforcement, such as particulate, whiskers, or chopped fiber, into an aluminum alloy matrix produces a composite with significantly higher yield strength, tensile strength and modulus of elasticity than the matrix alloy alone. However, the addition of silicon carbide whiskers to conventional alloys results in a composite with poor ductility and fracture toughness, and thus limited industrial application.
Several studies have suggested that the reason known silicon carbide whisker reinforced aluminum alloys have poor ductility and toughness is void nucleation at the whisker ends. The whisker ends are believed to be the sites of stress concentrations. Microstructural damage at these sites results in void initiation, interface decohesion, and whisker cracking. Eventually, there are sufficient openings created to form a fracture path. A 1986 study by S. R. Nutt entitled "Interfaces and Failure Mechanisms in Al-SiC Composites" made the above observations and concluded that since most sites at which damage is initiated involve the whisker reinforcements, there may be a fundamental limitation to the ductility of whisker reinforced aluminum alloys which cannot be overcome by modifications to the alloy content. Contrary to this generally accepted view, the present invention modifies the alloy content of the aluminum matrix to provide a ceramic reinforced aluminum matrix composite with ductility and fracture toughness superior to that of a composite using a conventional alloy matrix. Moreover, the composite of the invention achieves improved fracture toughness and ductility without a significant sacrifice of strength and stiffness.
Another previous alloy development program, which evaluated different, conventional, ceramic reinforced aluminum alloy matrices, agreed with the hypothesis that SiCw reinforcement dominates the failure process, and concluded that the matrix alloy has, at most, a minor role in determining the elongation to fracture. It was found that independent of the matrix alloy or temper, all high strength composites made with conventional aluminum alloys had elongations to failure of about 2.5%. It was thus believed that the strength and ductility of the composites could not be improved by using different aluminum alloys. Again, this previously accepted position is contrary to the findings of the present invention.
Previously known composite materials have used conventional heat treatable aluminum alloys, defined according to the Aluminum Association Classification System, as matrices for reinforcement by a ceramic material. One commonly used aluminum alloy is alloy 2124. 2124 consist essentially of 3.8-4.9% copper, 1.2-1.8% magnesium, 0.3-0.9% manganese, up to 0.2% silicon, and up to 0.3% iron. This alloy has generally been reinforced with silicon carbide whiskers. Because the silicon carbide used for reinforcement is discontinuous, this composite can be fabricated with conventional metal working technology.
Silicon carbide reinforced aluminum matrix composite materials are often known by the SXA.RTM. trademark. For example, SXA.RTM. 24/SiC is a composite of alloy 2124 reinforced with SiC. The strength and stiffness of extruded, forged or rolled SXA.RTM.24/SiC is significantly greater than existing high strength aluminum alloys. The light weight and improved strength and stiffness of SXA.RTM.24/SiC make it a useful material in many industrial applications. For example, it can improve the performance and reduce the life-cycle cost of aircraft. However, the ductility and toughness of SXA.RTM.24/SiC is too low for many aircraft components where damage tolerance and ductility is critical. This has prohibited the use of conventional ceramic reinforced alloys in aircraft and similar applications to which they would otherwise appear to be ideally suited.
Upon tensile loading, SXA.RTM. composite made with conventional matrix alloys, like 2124, fracture catastrophically without the onset of necking. In SXA.RTM.24/SiC.sub.w, examinations of fractured specimens have shown that fracture usually initiates at large particles having dimensions less than 50 um, such as insoluble intermetallic particles, coarse silicon carbide particulate contaminants which accompany the SiC.sub.w, and agglomerates of SiC.sub.w. Upon crack initiation, fracture propagates by a dimple rupture mechanism, where SiC reinforcement is the principle site for microvoid nucleation. One study of a composite made from alloy 2124 reinforced with 15 vol. % SiC.sub.w suggested that this fact implied that the large insoluble intermetallic dispersoids and constituent particles are fracture nucleation centers, and that the large variety of precipitates and dispersed particles within the matrix are the primary cause of the small strain to fracture. It was hypothesized that if the intermetallic dispersoids were removed, the fracture behavior would be dominated by the reinforcing fibers.
One type of large insoluble intermetallic particle formed in a composite made using a conventional alloy for the matrix is formed by transition elements, which are deliberate and necessary alloy elements in the unreinforced alloy. The transition elements serve to retain the best combination of strength, damage tolerance, and corrosion resistance. For instance, manganese is a critical addition to 2124, which precipitates submicron Al.sub.20 Mn.sub.3 Cu.sub.2 particles during the ingot preheat and homogenization treatment phases of preparing the alloy. These particles are generally referred to as dispersoids. The dispersoid particles are virtually insoluble and have a dual, but contradictory, role in unreinforced alloys. By suppressing recrystallization and grain growth, the dispersoids promote transgranular fracture which is associated with high toughness. However, dispersoids also promote fracture by nucleating microvoids and can thus reduce the transgranular fracture energy. Dispersoids like Al.sub.20 Mn.sub.3 Cu.sub.2 in 2124 are not amenable to the composite consolidation process typically used in making ceramic reinforced aluminum alloy matrix composites. The slow cooling rate from the liquid/solid hot press consolidation temperature destroys the homogeneous, rapidly solidified microstructure of the gas atomized alloy powder and allows large intermetallic constituent particles of (Mn,Fe,Cu)Al.sub.6 or Al.sub.20 (MnFe).sub.3 Cu.sub.2 to form in addition to the dispersoids.
Another type of insoluble intermetallic particle contains copper, an essential element which strengthens 2124 upon age hardening. The composition limits of alloy 2124 allow Cu to exceed the solubility limit of the Al-Cu-Mg system. Accordingly, x-ray diffraction has identified Al.sub.2 Cu after solution heat treating, cold water quenching and natural aging of the composite, SXA.RTM.24/SiC. When the copper bound to the compound Al.sub.20 Mn.sub.3 Cu.sub.2 is considered, approximately 3.9% copper (at the nominal composition) is available to precipitate the strengthening phases upon natural or artificial aging. At this concentration, the ternary Al-Cu-Mg solvus shows that undissolvable soluble constituents can exist in the composite, as shown in FIG. 1. Complete dissolution of the soluble phases is not possible at the maximum customary 920.degree. F. (493.degree. C.) solution heat treatment temperature for 2124, which is used to avoid eutectic melting.
It has been found, however, in accordance with the present invention, that dispersoid particles may not be needed in a reinforced aluminum composite because the reinforcement and dispersed aluminum oxide (which is an impurity introduced with the aluminum powder) appear to give adequate control of grain size. Thus, omitting insoluble metallic elements, such as manganese, from 2124, while retaining the elements needed for strengthening by age hardening, would eliminate the large intermetallic particles responsible for premature crack initiation. Omitting the dispersoids likely improves the fracture toughness of the composite by increasing the transgranular fracture energy of the matrix alloy. Since the amount of ceramic reinforcement is not changed, strength and stiffness of the composite are maintained.
In summary, ceramic reinforced aluminum alloy composites made with conventional alloys, such as 2124, form insoluble and undissolved soluble constituents which can not be eliminated by prolonged homogenization. These constituents are a permanently installed, deleterious component of the matrix microstructure. Thus, in accordance with the present invention, control of the type and amount of alloying is needed to eliminate the constituents which act as sites for crack initiation and propagation at small (2.0%-2.5%) strains.